1. Field of the Invention
The present invention relates to a fluorescent tube power supply and a backlight.
2. Description of the Related Art
Voltage and current sometimes change on a demand side due to inconsistency between a supply side that generates power and transmits power, and the demand side on which great change in load occurs. Electrical products may not operate properly when such change occurs.
For instance, in a backlight used in a liquid crystal television, a converter, that is, a direct current (DC) power supply for converting an output of a domestic commercial power supply to a DC and an inverter for converting an output of the DC power supply to an alternating current (AC) are used as a power supply circuit. As the current or the power input to the inverter reduces, the brightness of a screen becomes less than the preset brightness. As the current or the power increases, the brightness of the screen becomes more than the preset brightness.
In particular, a discharge tube such as a Cold Cathode Fluorescent Lamp (CCFL) used in the backlight has negative resistance characteristics. That is, the current decreases as the voltage rises, and the current increases as the voltage lowers. Impedance of a fluorescent tube such as a CCFL differs depending on the current-carrying time, and also varies depending on each fluorescent tube. The luminance of the fluorescent tube and the backlight, on the other hand, depend on a tube current or a tube power of the fluorescent tube, and thus a stable current supply and power supply are required for the fluorescent tube.
The fluorescent tube employs PWM control to regulate illuminance level. A width of a pulse that controls ON/OFF of the fluorescent tube, that is, a duty ratio is changed by performing the PWM control. When the duty ratio is changed, light-on and light-off of the fluorescent tube are repeated at a speed unrecognizable by humans, so that illuminance level can be regulated.
On the other hand, if the current or the power input to the inverter changes, the current or the power of the fluorescent tube does not stabilize in the PWM control and a state where the duty ratio of the PWM control becomes large sometimes continues.
Under such situation, Japanese Laid-Open Patent Publication No. 2006-79997 proposes a lighting device of a fluorescent tube lamp which has a triangular wave oscillator for increasing or decreasing an operation frequency of a switching element and a comparator for converting the triangular wave output from the triangular wave oscillator to a rectangular wave of a predetermined duty ratio and outputting the same arranged in a drive circuit for driving the switching element that carries out the PWM control. According to such a circuit configuration, the power accumulated in a transformer is made substantially constant by fluctuating the operation frequency of the switching element while maintaining the duty ratio of the input power supply substantially constant, and so that the energy accumulated in the transformer during the ON operation of the switching element cancels out the voltage fluctuation of the input power supply.
Japanese Laid-Open Patent Publication No. 2002-153075 proposes a power supply device for controlling VP−P (difference in maximum peak voltage and minimum peak voltage) of an output waveform, a frequency, and a duty ratio by arranging a switching circuit for controlling a rise of a secondary side output of a transformer and a switching circuit for controlling a fall of the secondary side output of the transformer.
Japanese Laid-Open Patent Publication No. H05-49252 proposes an inverter for detecting a secondary side output of a transformer, generating data to set a duty ratio of a switching pulse by feeding back the detected secondary side output, comparing the generated data and the detected secondary side output, and changing the duty ratio based on the comparison result to have the secondary side output at an appropriate value.